Gh action fix

[ms888ekb]

Fix for the unit tests that failed:

1. Changed the way train and validation tiles are defined. Now it has more stable and accurate logic.
2. Fixed test_gh_action_ci.py. It failed because it didn't have access to the sources it tried to verify, such as:
   the "data" folder and all test tifs (which are archived, initially).
3. Minor fixes.

[LucaRom]

Great work, thank you!

[CharlesAuthier]

Thanks